The present invention relates to a process for collecting process water at a paper machine or the like, wherein water from several suction elements provided at the wire is led in such a way that an essentially unbroken liquid column is maintained in the respective outlet pipes of the respective suction elements and said liquid column is made to join common collecting vessels. The invention also relates to an arrangement for collecting process water at a paper machine or the like, said arrangement comprising suction elements provided under the wire section said suction elements being connected by essentially vertical outlet pipes to at least one collecting vessel.
In the case of prior known papermachines and the like machines having suction elements in the form of suction boxes, the outlet pipes from said elements are generally connected to an essentially open collecting vat, which at the same time often functions as a deaerating means for the air contained in the water. The process water is generally at least partially returned from this collecting vat for joining a suitable phase in the initial part of the process in order to re-use a part of the material and the energy remaining in this water and also in order to reduce the amount of effluents from the process.
Since this so called long system should be able to balance the process both at start-up and during normal operation as well as in case of any operational disturbances, the system normally comprises huge amounts of liquid, which, on the other hand, results in a slow circulation and a process with a tendency to contamination, Since this long system extends over a large portion of the machine, it generally includes many long extensions with inclined or horizontal pipes. Practice has shown that these pipes often create problems since the air which always remains in the water easily forms air pockets which, in turn, results in an instable process with disturbances in pumps and other equipment. The air also causes unwanted foaming which is deposited on free surfaces in tanks and pipes and creates additional disturbances. In order to alleviate these problems, the system generally includes special deaerating means for removing as much air as possible from the water in order to enable pumping to different parts of the machine. In practice it is, however, difficult to make these deaerating means sufficiently effective without at the same time creating disturbances in the actual liquid flow. Therefore, a significant amount of unwanted air will in practice always be present in the process water.
Since the long system comprises said huge volumes and long pipe lines, it is in practice also difficult to maintain uniform flow conditions at all points. Stagnant water is thus a common problem and it results in accumulations of dirt in dead angles of the system. This dirt contributes to disturbances by occasionally breaking loose and by following the liquid flow in the form of accumulations which then choke up pumps and valves and which cause quality problems in the actual product.
In order to maintain the function in the suction elements it is important that the liquid column in the barometric outlet pipes of the suction boxes is kept unbroken. In the prior art this has been solved by connecting the pipes to one or more substantially open lock water tanks from which the water is pumped to the process. In practice the tanks have to be big since the pumps need a certain inlet pressure in order to function satisfactorily. At the same time the huge volumes cause problems with contamination and they also complicate quick changes in the production, such as changes in quality and the like. Since the pipe lines in practice include long extensions, it is also difficult to avoid the formation of air pockets which interrupt the vacuum and cause operational problems. The voluminous systems in themselves also cause troubles since it is not always easy to find suitable places for the space consuming systems.
The object of the present invention is to provide a solution to these problems and to provide an arrangement with a small liquid volume, little contamination risks and a minimal part of free liquid surfaces. At the same time the object is to provide an effective deaeration and a reduction in the drawbacks which are caused by an unwanted foaming. The arrangement should also offer simple and effective means for controlling the process.
Said objects are provided according to the invention in the way disclosed in the appended claims. Thus, the process according to the invention is characterized in that the liquid levels of the outlet pipes are regulated by regulating the liquid level in at least one separate level vessel towards which the water from the respective outlet pipes is led in at least one closed and essentially horizontal collecting channel which is common for several outlet pipes in such a way that the liquid in said collecting channel at the same time forms the lower part of said unbroken liquid column. The arrangement according to the invention is characterized in that the collecting vessel is composed of at least one essentially closed and generally horizontal collecting channel to which said outlet pipes connect below the channel liquid level.